Illustration of the X-sigma method. Shown are gas density values of the relaxed cluster CL1 in an arbitrary, thin spherical shell of width Δr ≈ 11 kpc between r₅₀₀ and r₂₀₀ (this is an example choice for illustration only). The dashed line shows the median density, while the blue solid line shows the threshold for X = 3.5. The red solid lines show further arbitrary example choices (X = 2, 3, 4) to give an impression of the extent to which variations of order unity in X affect the distribution.